[REL] v0.6.5 final cleanup

[HaoZeke]

Fixes #1520. Closes #1517. Closes #1518. Last one before the release :)

[mattip]

LGTM, pending CI passing

[HaoZeke]

Sorry one last PR incoming, currently the rattler backend seems to be conflicting with version pin for libmambapy. Just going to remove libmambapy altogether.

[DimitriPapadopoulos]

Bump 0.6.4 → 0.6.5 in docs before releasing:

$ grep -R 0.6.4 docs/
docs/source/dev.rst:       pipx run towncrier build --version 0.6.4 --date "$(date -u +%Y-%m-%d)"
docs/source/dev.rst:       git commit -m "REL: Finalize v0.6.4 log [wheel build]"
docs/source/dev.rst:       git tag -a v0.6.4 -m "Version 0.6.4 release"
docs/source/using.rst:- ``version``: Version of the ``asv.conf.json`` spec. As of ``v0.6.4``, there is only version ``1``.
$ 

[lucascolley]

I think we also want to mention py-rattler in the changelog, unless I've missed it?

IMO 0.7.0 might be appropriate due to that, too.